Recently, in the automotive fields, there is demand for more lightweight vehicle bodies in order to achieve lower fuel consumption and a reduction in CO2 emissions, and there is also demand for higher vehicle body rigidity in order to improve collision safety. In order to meet these demands, the need to employ high strength steel sheets in vehicle bodies, components, and the like is on the rise.
Processes in vehicle body assembly, component attachment, and the like predominantly employ spot welding. However, there have hitherto been issues regarding the tensile strength of joints, particularly when spot welding is used on high strength steel sheets having a high tensile strength.
There is a large amount of carbon etc. contained in the base material of a high strength steel sheet in order to attain its strength. Moreover, in spot welding, rapid cooling is performed immediately after heating the weld portion, such that the weld portion adopts a martensite structure, increasing the hardness of the weld portion and a heat affected zone, and reducing ductility.
In spot welding of high strength steel sheets, methods using a two-stage welding, in which a post heating welding is performed after a main welding, exist as methods to improve spot weld portion ductility and secure joint strength.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2002-103048, a tempering welding is performed once a fixed duration has elapsed after a spot welding has ended. The spot weld portion (a nugget portion and a heat affected zone) is annealed, reducing the hardness of the weld portion. JP-A No. 2010-115706 describes a method in which, after forming the nugget with a main welding, a post heating welding is performed with a current value of the main welding current value or greater.